336 BENE ST WAREEN. 



the purpose of firmly fastening the hydroid to a support. If 

 the suckers were for the sake of anchoring the hydroid, the 

 purpose woukl be much more effectively acccomplished by the 

 pits being lined by perisarc, but as a rule there is no perisarc 

 towards the base of the pit. 



In the immediate neighbourhood of the sucker, both in the 

 ectoderm and endoderm, large rounded cells (fig. 32, y.c.) filled 

 with yolk-like globules are found. It is possible that these 

 represent the nutritive substances obtained from the alga. 

 The tissue of the alga is very hard, being thoroughly impreg- 

 nated with calcium carbonate, but the sucker can, in some way, 

 eat into it. The tissue of the alga around the suckers is 

 curiously stained for some depth, indicating that some sub- 

 stances are secreted by the hydroid and passed into the plant ; 

 perhaps some acid for dissolving the calcium carbonate is 

 formed. 



The cross-section of the coenosarc shows an endodermal 

 tube {cce.), above which is a crumpled layer of ectoderm [ect.), 

 from the edges of which a sheet of ectoderm passes round in 

 contact with the 23erisarc {e.s.) and leaves a distinct cavity 

 {ec.c). In the spaces below the crumpled ectoderm {ect.) 

 numerous loose granules {gr.) may often be seen. 



The response of the plant to the stimulus, due to the j)ene- 

 tration of the suckers, is an attempt to envelop the hydro- 

 rhiza and pinnate stems; the cortex of the alga grows up 

 around them (text-fig. 17, C and D, e.C), and may ultimately 

 completely surround the hydrorhiza, as at (j.h. 



It would appear that the complete envelopment of the 

 hydrorhiza by the tissues of the alga acts prejudiciously on 

 the coenosarc, since the ectoderm is found to have dwindled 

 almost entirely away, and the endoderm tube is reduced to 

 the smallest dimensions (vide fig. D, oJi.). 



There is thus a struggle between the plant and the hydroid ; 

 the host attempts to smother the parasite by growing over it, 

 and the spread of the hydroid nuist be considerably checked 

 thereby. 



